syscalls: rename __ZEPHYR_KERNEL__
syscalls: rename __ZEPHYR_KERNEL__

__ZEPHYR_SUPERVISOR__ more accurately represents what this means: that
the code is intended for scenarios when the CPU is expected to be
running in supervisor (privileged) mode. This could be in the kernel or
in the application.

Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>

explora26/zephyr,nashif/zephyr,explora26/zephyr,nashif/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,finikorg/zephyr,ldts/zephyr,punitvara/zephyr,aceofall/zephyr-iotos,fbsder/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,fbsder/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyriot/zephyr,finikorg/zephyr,fbsder/zephyr,kraj/zephyr,kraj/zephyr,finikorg/zephyr,zephyriot/zephyr,punitvara/zephyr,kraj/zephyr,galak/zephyr,Vudentz/zephyr,aceofall/zephyr-iotos,zephyrproject-rtos/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,ldts/zephyr,mbolivar/zephyr,zephyriot/zephyr,punitvara/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,Vudentz/zephyr,fbsder/zephyr,kraj/zephyr,kraj/zephyr,galak/zephyr,explora26/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,galak/zephyr,punitvara/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyriot/zephyr,ldts/zephyr,fbsder/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr